Evaluation of blood-warming devices with the apparent thermal clearance

Hypothermia (less than 35 C) associated with massive transfusions is a serious clinical problem. Maintenance of normal temperatures in this setting is essential and requires the use of rapid and efficient blood-warming devices. The optimal design of such units should include a low priming volume, large heat transfer area, low pressure drop, the ability to operate at high flow rates, and the capability of warming blood from 4 to 32 C, or higher, without untoward effects. We have previously formulated a theoretic model for blood warmers and described a measure of the intrinsic efficiency of these devices, the apparent thermal clearance (VT,cl). In this study, we tested four different types of commercially available blood warmers (the single-coil immersion heater; the single-channel dry wall electric heater; the multichannel countercurrent heat exchanger; and the single-channel, countercurrent heat exchanger) and determined the apparent thermal clearance values using saline and packed red blood cells. The results indicate that only the single-channel countercurrent unit is efficient enough to meet the established requirements. The apparent thermal clearance provides a useful and rigorous means for comparing and evaluating blood-warming devices.